A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electro catalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
In proton exchange membrane (PEM) fuel cells, the electrolyte is a solid polymeric membrane. The membrane is electronically insulating but ionically conducting. Proton-conducting membranes are typically used, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to create water.
The principle component of a PEM fuel cell is known as a membrane electrode assembly (MEA) and is essentially composed of five layers. The central layer is the polymeric membrane. On either side of the membrane there is an electrocatalyst layer, containing an electrocatalyst, which is tailored for the different requirements at the anode and the cathode. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer, must allow products to be removed from the electrocatalyst layer, and must conduct the electric current that is generated by the electrochemical reactions. Therefore the gas diffusion layer must be porous and electrically conducting.
Direct methanol fuel cells are a promising alternative power source for portable power applications and electronic devices such as mobile telephones and laptop computers. Methanol is a readily available fuel that is easy to store and transport and has a high energy density. Methanol or a mixture of methanol and water is supplied to the anode, and an oxidant, usually air or oxygen, is supplied to the cathode. The anode and cathode reactions are shown in the following equations:Anode: CH3OH+H2O→CO2+6H++6e−Cathode: 3/2O2+6H++6e−→3H2O
The state-of-the-art anode electrocatalyst is a platinum-ruthenium alloy, which may or may not be supported on a conducting support material such as carbon particles. Platinum is an expensive metal and the present inventors have sought to provide an anode electrocatalyst that has useful activity in direct methanol fuel cells, but uses less or no platinum and is therefore less costly to produce. The present inventors have developed palladium-ruthenium alloy catalysts that have surprisingly high activity in direct methanol fuel cells.
U.S. Pat. No. 6,995,114 discloses platinum-ruthenium-palladium catalysts that are useful in direct methanol fuel cells and U.S. Pat. No. 5,208,207 discloses platinum-ruthenium-palladium catalysts that are useful in fuel cells wherein the fuel is reformats. The disclosed ranges specify that the catalysts must contain at least 10% platinum (expressed as atomic percentage) and all the examples contain at least 20% platinum. The present inventors have found that a palladium-ruthenium alloy catalyst (containing no platinum or less than 10% platinum) has useful activity as an anode electrocatalyst in a direct methanol fuel cell. By contrast, a palladium-only catalyst and a ruthenium-only catalyst do not have useful catalytic activity. U.S. Pat. No. 6,995,114 contains a comparative example (electrode 13) which is said to represent a 50:50 Pd:Ru alloy. This electrode exhibits no activity for the electrochemical oxidation of methanol, so the present inventors assume that this catalyst is not a palladium-ruthenium alloy, but an inactive mixture of palladium and ruthenium.